Loose end from r5031: the Kex panel should only be displayed in

Loose end from r5031: the Kex panel should only be displayed in
mid-session if we are not using SSHv1. I've done this by introducing
a generic `cfg_info' function which every back end can use to
communicate an int's worth of data to setup_config_box; in SSH
that's the protocol version in use, and in everything else it's
currently zero.

[originally from svn r5040]
[r5031 == d77102a8d535a7000d6d909529a61a1564f6d678]